The use of innovative technologies to improve treated wastewater irrigation of olive trees in the Souss-Massa region, Morocco

The natural water resources in Morocco are among the lowest in the world. Their potential is evaluated at 22 billion m³ per year; the equivalent of 700 m³/inhabitant/year. The yearly surface water resources are evaluated at 18 billion m³ on average, but vary from 5 to 50 billion m³/year from year to year, and basin to basin (ABHSM 2016). This difference becomes a matter of concern because not only it occurs in time but also in space. One example is the northern Moroccan basins which cover only 7% of the country's territory but hold more than half of the water resources (ABHSM 2016). In southwestern Morocco, the Souss-Massa basin covers an area of 27,800 km² approximately and most of the available water is consumed for agricultural purposes (86%), and the remaining is consumed by industry and potable water (Mansir et al. 2018).

Souss-Massa basin is characterized by an arid climate with low and highly variable precipitations; the annual rainfall can be as low as 15 mm in a dry year and as high as 600 mm in a humid year with an average amount of 200 mm/year (ABHSM 2016; Abahous et al. 2018). But it is needed to be aware that these basins could not meet the growing demand for agricultural production for a long time even though the region mobilizes around one billion cubic meters of surface and groundwater generating a deficit of 290 million m³/year. As long as this negative balance is covered by groundwater mining, the need to look at alternative water resources has not been a priority or at least up to date when this practice led to a lowering of its piezometric level with an average of 2–3 m per year (Choukr-Allah et al. 2017; Attar et al. 2022).

In addition, changes in precipitation accompanied by successive droughts generate long-term impacts on water availability for farmers (Aziz & Farissi 2014). Therefore, different measures should be taken to reduce the overexploitation of the fresh water (FW) resources (Mansir et al. 2018; Abahous et al. 2018).

In this regard, wastewater can be an opportunity even though such problems are not by no means trivial. If on one hand, unmanaged wastewater could generate pollution and risks to human health and ecosystems, on the other hand, its safe reuse can be a source of several potential benefits (Hernandez-Sancho et al. 2015). Reuse of treated wastewater (TWW) preserves high-quality, expensive freshwater for the most important uses, mainly drinking. In other words, collecting and treating wastewater protects existing sources of value as FW, the environment in general, and public health could also be used to recharge aquifers. If managed properly, TWW can be a source of agricultural water, but not less than some FW sources (Mansir et al. 2021; Guemouria et al. 2023). In this complex context, a comprehensive evaluation of both costs and benefits is required to assess the feasibility of the TWW reuse option.

In 2011, for instance, the wastewater production in Morocco was 700 million m³, from which only 25% (177 million m³) was treated and an even smaller portion was used (80 million m³) (Guardiola-Claramonte et al. 2012). However, this use is mainly localized to the periphery of some large interior cities where agricultural lands are located downstream of treated effluent (Marieme et al. 2021).

Yet, the use of TWW in citrus irrigation in Morocco along with adequate public incentive policies can lead to saving freshwater up to 3,580 m³/ha. Moreover, in terms of fertilizers, TWW can cover up to 81 and 38% of the crop requirements in nitrogen and phosphorus, respectively (Bouchaou et al. 2017; Oubelkacem 2018). For instance, in the region of Souss-Massa, ten treatment plants are largely used in agriculture, landscaping, and golf course irrigation (Choukr-Allah & Hamdy 2005; Belabhir et al. 2021).

Many studies explored the effects of TWW for irrigation on cereals, forage, and vegetable crops in Morocco observing yields were higher for plants irrigated with TWW as was water use efficiency (Choukr-Allah & Hamdy 2005).

Among the various crops, however, olive represents one of the main cultivations in Morocco, the area covered exceeded a million hectares as of 2016 (MAPM 2019) which got the country to rank fourth worldwide in terms of area occupied (FAOSTAT 2017; Elkouk et al. 2022). This grove is also known for its relatively high salt and boron resistance which makes it tolerant for irrigation with TWW (Erel et al. 2019). In fact, Bedbabis et al. (2010) and Erel et al. (2019) reported higher yields were achieved under TWW irrigation management compared to FW.

However, long-term reuse of TWW may induce a significant loss of production, thus, adopting appropriate irrigation management becomes of primary importance.

One option to design a proper irrigation scheduling could be to adopt routine monitoring and modeling in order to control and predict the effects of treated wastewater irrigation on soil, and agricultural productivity. The modeling may be a relevant tool to properly estimate the crop water requirement and preserve soil quality. In this regard, several models were developed with the aim of supporting decision-makers with TWW irrigation management. Among them, the safe irrigation management (SIM) model is one developed and here tested to define TWW irrigation management for citrus in Morocco.

Alongside, improving the performance of the irrigation systems is also required to deal with the fairly common fouling and clogging problems that commonly occurred with TWW irrigation, consequently are proposed alternatives. For the time being, commercially available drippers are compared to assess the main clogging problems with TWW irrigation. (Ravina et al., 1997). It was observed that the dripper clogging hazards are mainly due to microbial activity, and influenced by the discharge rates, in particular, highest discharge rates have shown smaller problems than lower discharge rates (Ravina et al. 1997).

Based on the above statements, studying the effects of using TWW as a source of irrigation water for olive trees seems evident, along with setting up a drip irrigation system able to contain the clogging problems as long as the irrigated olive cultivation can be enhanced with TWW.

In this regard, a study was conducted to evaluate the feasibility of the irrigation of olive trees by TWW using two technologies: an innovative nozzle (IN) and a treated wastewater irrigation management model (SIM). A field experiment has taken place to compare two irrigation schedules: model and crop evapotranspiration demand using two different water qualities (freshwater and TWW) and combined two practices: IN and the classical drip irrigation setting.

The experimental design took place in March 2019 and involved an olive Moroccan traditional variety ‘Picholine Marocaine’. This characterizes both its table olives and olive oil quality its high adaptability to the local weather conditions. It is the most important variety in Morocco occupying over 90% of the surface (El Mouhtadi et al. 2014), even though it can be sensible to certain diseases and alternate bearing.

The Picholine Marocaine is not usually used in intensive setups, but rather to densities between 80 and 100 trees/ha. Overall, modern orchards' density setups are around 400 trees/ha (El Mouhtadi et al. 2014).

A plot of 450 m² was delimited and used to house 60 olive trees. Four-year-old trees (cv. picholine) were transplanted in March 2018. The high density of 1,333 trees/ha was achieved using distances of 2.5 m between the lines and 3 m between the trees on the same line. Each line included five trees with 12 lines overall, and divided them into four groups of which each one is associated with one treatment.

The trial was arranged in a randomized complete block design. Two treatments were: FW and TWW, and combined to two innovative practices: irrigation management support (SIM model) and irrigation device (IN).

The randomized block is divided into

• – one factor (irrigation type) with four levels (TWW + SIM, TWW + IN, FW + IN, FW + DRIPPERS),

• – three blocks (repetitions),

• – each block with four experimental units resulting in a total of 12,

• – each experimental unit containing five trees.

In detail, four treatments selected for the experimentation are: (1) the irrigation management support and irrigation device (T1 – TWW + SIM + IN), only the irrigation device (IN) and TWW (T2 – TWW + IN) or with FW (T3 – FW + IN), and finally treatment with neither the technology nor the practices (T4 – FW + DRIPPERS) which represented the current farmers' practice (Table 1).

The TWW used for the irrigation purposes comes from the L'Mzar treatment plant. The TWW is characterized by high electrical conductivity (EC) and is very rich in nitrogen, which exceeds 3,000 μS/cm and 35 mg/l, respectively.

As for the FW used for T3 – FW + IN and T4 – FW + DRIPPERS treatments, it is supplied using the Multi-Service Autonomous Board of Agadir (RAMSA).

Two methods used to determine the crop water requirement in this experiment:

• – Crop evapotranspiration based on the single crop coefficient method (FAO 56)

• – SIM irrigation scheduling

The olives crop coefficients were obtained according to Fereres et al. (2011), which calculates the ET_c of olive trees by computing the only evaporation rate from the wet emitter's bubble (Table 2):

SIM is a one-dimensional daily water and salt balance model used to simulate the crop water requirement accounting for crop, soil parameters, water quality, and climate.

The model contains four modules:

• (1) Crop water demand and irrigation scheduling;

• (2) Salinity management;

• (3) Bacterial movement and risk assessment;

• (4) Nutrient management.

One of the benefits of using TWW for irrigation is that it comes with a set of nutrients readily available to the plants. As such, no commercial fertilizers were added to the plots T1 – TWW + SIM + IN and T2 – TWW + IN.

On the contrary, to cover the olive nutrient requirement for the treatments: T3 – FW + IN and T4 – FW + DRIPPERS, a fertilization plan was set and in Table 3, the amounts of fertilizers are listed:

To closely follow the effect of irrigation with TWW on olive trees, agro-physiological parameters were monitored in situ from November 2019 to September 2020. Nine trees per treatment were selected to assess the effects of irrigation practices on olive growth and soil response. The collected parameters are described below:

• Plant height: measured using a stick vertically from the highest leaf to the ground. The height was monitored to trees and the four treatments throughout the experiment. The height was obtained as the distance between the highest leaf and the ground (Häusler et al. 2012).

• Canopy dimension: to obtain the canopy diameter throughout the experiment year, two perpendicular diameters (North/South and East/West) were measured and averaged.

• Shoot growth: measured by selecting two shoots on opposite sides of the canopy and regularly recorded once a month from November 2019 to December 2020, with eighteen values for each treatment.

• Trunk diameter: the trees were marked at the beginning of the experiment at 20 cm above the soil. The trunks were then measured at the same marked level.

• Photosynthesis rate: measured by a portable infrared gas analyser (IRGA) called LCi-SD. Using the LCi-SD plant leaf chamber, nine exposed leaves per treatment were taken between 10 am and 12 am (Greenwich Mean Time (GMT) +1) (Neales & Incoll 1968).

To detect the nutritional traits of leaves, chemical analysis, which includes total nitrogen and minerals was also carried out and based on the methodology described by Fernandez-Escobar (2010). 2 leaf samples were collected from non-fruit-bearing shoots in July (at the last field campaign); afterward, they were oven-dried at 70 °C for 72 h and milled to powder. The total nitrogen was determined using the Kjeldhal method, while other elements (phosphorus, potassium, magnesium, calcium, zinc, and sodium) were determined using the inductively coupled plasma optical emission spectrometry (ICP-OES) (Fernandez-Escobar 2010).

In addition, 48 soil samples at a depth between 0 and 30 cm were taken in November 2019, March 2020, July 2020, and September 2020 (12 samples each month). The analyses were done to determine the content of nutrients and assess the impact of both irrigations (TWW and FW + fertilizer), on the physicochemical properties of the soil. The analyzed parameters were pH; soil solution EC; sodium (Na); phosphorus (P); mineral nitrogen (⁠⁠, ⁠), and potassium (K).

Soil pH, EC, and phosphorus were determined according to the methods of NM ISO 10390, NM ISO 11265, and NM 11263, respectively, and total Kjeldahl nitrogen using the NM EN 16169 method (Ciesielski & Sterckeman 1997). The Sodium, potassium, magnesium, and calcium content of the soil samples were determined using the NF X 31-108 method (Ciesielski & Sterckeman 1997).

The statistical analysis was carried out by the ‘MINITAB’ software. Mean separation was done using the Tukey method at a 5% confidence level.

In order to evaluate the economic viability and performance of the different wastewater reuse options, a cost–benefit analysis (CBA) was performed for the different treatments for a 30-year project lifetime. The specific costs related to the adoption of the different technologies and the associated practices have been distinguished into investment costs (emitters, pipes, filtration, pumps, control panel, etc.), operational expenses: energy, resources, labor, and maintenance costs. The benefits are mainly due to savings (on water and fertilizer costs) and yield increase. As for the savings on water and fertilizers costs from the use of TWW and IN and the SIM software, they were extrapolated from the field study, while a sensitivity analysis tested the economic performance of the projects using different yield increase rates. A time frame of 30 years and an interest rate of 5% have been considered to calculate two indexes – the net present value (NPV) and the benefit-cost ratio (BCR).

As shown in Supplementary Table 4, in general, the pH of TWW is slightly alkaline. Therefore, if the TWW is applied to soils with appropriate alkalinity, the acid/base balance of the soil is not disturbed. While pH values below 5 or above 8.5 affect the growth and survival of soil microorganisms (RAMSA 2017). In this study, pH values were close to neutral which is acceptable for watering green areas according to WHO (Bulletin Officiel 2002).

Regarding the water's EC, its value ranged from 3,000 to 4,530 μS/cm. These high values also explain the high concentrations of sodium and chloride in the water. As a consequence, the high level of salinity has a negative influence on the soil solution, increasing its osmotic pressure and preventing seed imbibition and root absorption. Moreover, high levels of salinity, Chemical oxygen demand (COD), BOD_5, and SS are recorded in the raw water due likely to the fish farms which contribute to the discharge tons of brine and organic matter into the city's sewage system.

Although treatment performance is generally very satisfactory, concentrations of soluble salts in wastewater effluent were very high (RAMSA 2017). The high salt content of L'Mzar Station's TWW is mainly due to salt discharges from the various fish canning factories. This source of salts, if not controlled, may pose serious problems that can limit the use of treated water on salt-sensitive species.

Considering the high concentration of salts in the TWW, with water EC around 4,000 μS/cm and Chloride levels up to 850 mg/l, means the treatment plant provides a low-quality and below the reuse standards, which are equal to 3,000 μS/cm for conductivity and 100 mg/l for chlorides, respectively.

To assess the performance of the irrigation system, Distribution Uniformity (DU) was determined averaging the lowest quarter (25%) of the discharge sample to the global average (ASABE 2014).

Figure 4 shows the values recorded for the first three treatments (using IN) which are closer to the upper end of the recommended range for a point emitter, close spacing, and uniform topography that represent the situation in our field experiment. The drippers recorded 84% and this can be explained in part by the technical problems that occurred early in the experiment (81% average before January). The pump used for the treatment was oversized, so the pressure in the system was out of control, which led to irregularities in the dripper discharges. After solving the problem, DU recovered and averaged 85% between February and September.

Supplementary Table 5 illustrates the average values of DU coefficients. The treatments including IN and SIM models, showed significantly higher distribution uniformities (p = 0.001) with an average of 90% and to both water qualities (FW and TWW) compared to 83% obtained to the treatment with commercial dripper and FW.

Figure 5(a) shows the trees' height variation, observing approximately 40 cm during the 10 months between April and July which corresponds to the active vegetative growth phase of olive trees (Alfei et al. 2008). Afterward, growth is reduced as the tree enters a phase of reduced vegetative growth from July onward, during which energy is concentrated on fruit formation (Figure 5(a)).

The analysis of variance showed no significant differences between the four treatments at any date. This means that the trees grow uniformly and independently from the water quality, the technology, and the practices used (Majdoub et al. 2014).

Similarly, to the tree height, it can be observed in Figure 5(b) that the trend of the active vegetative growth phase significantly increases between April and July, followed by a decrease during the reduced vegetative growth phase of August–September. The canopy diameter increased by almost 40 cm for all treatments.

The Analysis of Variance (ANOVA) analysis showed no significant differences between treatments for the canopy diameter at any of the dates. Therefore, the canopy diameter was not affected by the treatment applied to the trees.

Figure 5(c) depicts the evolution of shoot growth throughout the experiment. During the first 5 months (from December to April), the shoots grew by 5 cm. The next 3 months (from May to July) witnessed an increase of 9 cm on average for the shoots, which corresponds to the active vegetative growth phase (Qadir et al. 2017). During the last 2 months (August and September), the increase averaged less than 2 cm.

The statistical analysis of these results showed no significant differences between treatments. Shoot growth was similar for the four treatments all along explained by the fact the trees are still young and with only a year of TWW irrigation supply.

Figure 5(d) shows the increase in tree-trunk diameter over time. It may observe a slow growth at the beginning of the experiment (1.5 mm between December and March), then swift from April until September (8.5 mm more on average). Overall, the trunk diameters for all of the olive trees increased by 1 cm during monitoring.

ANOVA showed that the differences between averages are highly significant (p = 0.011). Precisely, the photosynthesis rate for the treatment using TWW and no SIM (T2 – TWW + IN) was higher than the treatments (T3 – FW + IN and T4 – FW + DRIPPERS).

The campaign of 10 September 2020 shown in Figure 6(b), showed no differences between the treatments (p = 0.344).

Supplementary Tables 6 and 7 illustrate, respectively, the concentrations of the different leaf nutrient contents for the four treatments and the comparison between the recorded leaf mineral values and those referenced (Fernandez-Escobar 2010). According to these results, no significant differences were observed for any of the elements (p-values all greater than 0.05). This shows that, at the end of the experiment, the trees similarly uptaken the elements from the soil regardless of the applied treatment.

Based on the reference values set by Fernandez-Escobar (2010), the concentrations are in the adequate range for all of the elements except for potassium, which is a little short of the adequate range; for all treatments; but not low enough to cause deficiencies (Markhali et al. 2020).

According to the results obtained (Figure 7(a)), irrigation with TWW induced an alkalization of soil. The pH of the soil samples was within the range of 8.85–9.12 for soil irrigated with TWW (T1 – TWW + SIM + IN and T2 – TWW + IN) and 8.27–9.09 for soil irrigated with FW (T3 – FW + IN and T4 – FW + Dripper).

An increase in soil pH is noticed in the two treatments irrigated with TWW (T1 – TWW + SIM + IN and T2 – TWW + IN). Since soil pH behavior depends on the irrigation water quality, therefore the alkaline values are because TWW is rich in bicarbonates and sodium (Nobel & Cui 1992). A previous work carried out by other researchers indicated that long-term wastewater irrigation has an inconsistent impact on soil pH. Schipper et al. (1996) showed that continuous wastewater irrigation increases pH due to the high contribution of cations contained in the TWW. Others observed a decrease in pH due to the formation of organic acids and high ammonia content in the soil (Xu et al. 2010).

On the other hand, it was observed a decrease in the pH value for soils irrigated with FW. This decrease can be explained by the regular application of phosphoric acid as a fertilizer from the beginning of the vegetative stage and floral induction in March.

Statistically, ANOVA results show a very high significant difference (p = 0.000) between soil irrigated with FW and that irrigated with TWW, but not significant between the two specific irrigation devices: commercial dripper and IN.

Statistical analysis observation of the EC in March showed a very significant difference between treatments irrigated with FW and those irrigated with TWW (p = 0.001) with two homogeneous groups (Figure 7(b)). It should be noted that the T2 – TWW + IN treatment recorded the highest EC value at the end of the trial period, which was equal to 313 μS/cm, in contrast to the T3 – FW + IN treatment, which recorded the lowest value. This means that the IN did not contribute to changing the EC which may be due to the short period of monitoring and the fact that the irrigation systems did not show significant problems like clogging.

These results are similar to those found by Tsigoida & Argyrokastritis (2019), who showed that irrigation with saline water causes an accumulation of salts in the soil, this accumulation is directly related to the salinity level of the water used for irrigation. According to Jerate (1997), chemical analyses of soil and drained solutions showed the effect of wastewater salinization on the receiving environment, since the EC and salt content in the soil increase following irrigation with wastewater, which is confirmed by the rates of change of EC in the soil during the experiment.

However, the olive trees are young and have been irrigated with TWW for only one irrigation season; the salinity has not yet been pronounced. In fact, the value of EC was not different and averaged around 250 μS/cm which is still a low value. The soil type has also played an important role. As it is a sandy soil olive trees have not yet developed a strong root system, the salt was not taken up but rather accumulated below the soil root zone.

This can be demonstrated by looking at the results observed in terms of sodicity. In fact, for T1 – TWW + SIM + IN treatment the sodium value decreases over the irrigation season except for July 2020. This is due to the high evapotranspiration demand and the amount of supplied water higher compared to the remained period. It then decreases at the end of the irrigation season even if not significantly, because the sodium is a cation and it gets adsorbed to the solid particles of the soil.

With regard to sodium (Figure 7(c)), both treatments T1 – TWW + SIM + IN and T2 – TWW + IN showed 0.191 and 0.263 g/kg, respectively, at the end of the experiment. While treatment T3 – FW + IN recorded the lowest value of 0.113 g/kg.

In the T1 – TWW + SIM + IN treatment, the sodium concentration is lower than in the T2 – TWW + IN treatment that is 12%, due to the different amount of water used for irrigation management. These results can be explained by the fact that the high sodium content present in the wastewater contributed to an increase in the salinity through the soil profile compared to untreated soil conditions (Herpin et al. 2007). However, it shows at the end of the irrigation season that the sodium decreases for T1 – TWW + SIM + IN treatment compared to the same treatment TWW, without the SIM technology (0.191 and 0.263 g/kg, respectively).

The statistical analysis done to soil sodium content measurements showed that there was a significant difference in March and July 2020, between treatment irrigated with FW (T1 – TWW + SIM + IN and T2 – TWW + IN) and treatment irrigated with TWW (T3 – FW + IN and T4 – FW + DRIPPERS) (p = 0.002), but there is no difference between the two irrigation devices: commercial dripper and IN.

In Figure 7(d), nitrate concentrations in the soil increased to the four treatments and over time. For the T1 – TWW + SIM + IN and T2 – TWW + IN treatments, the increase is due to the TWW characteristics, basically nitrogen-rich compared to the FW. For FW, Ammonitrate was added as a fertilizer in March, April, and July 2020. The decrease in September 2020 is due to the nitrification of ammonium in nitrate that made available nitrate by the trees. This has been confirmed by the results obtained from the leaf analysis. In other words, the T2 – TWW + IN treatment allowed to uptake of nitrate efficiently.

Statistical analysis corresponded to the September 2020 data revealed a non-significant difference between the four treatments (p = 0.752), due to the small variations of nitrogen values observed and the short observation period of the olive response.

From a general point of view, the largest accumulation of soil nitrogen was observed between November 2019 and September 2020 to the TWW + SIM treatment, with an increase of 0.067 g/kg, as opposed to T3 – FW + IN and T2 – TWW + IN, which showed no difference over the same period.

Another factor to consider is that the nitrogen was supplied at a given TWW irrigation event compared to the fertigation practice adopted under FW irrigation at specific periods.

Concerning the phosphorus concentrations, statistical analysis showed no significant differences between the four treatments (p = 0.245) in September. Unlike a high significant difference (p = 0.000) was observed between treatments using IN (T1 – TWW + SIM + IN, T2 – TWW + IN and T3 – FW + IN) and that commercial drippers in March.

It is noted that treatments T4 – FW + DRIPPERS showed an increase from 58.98 mg/kg in November to 159.18 mg/kg in March. T2 – FW + IN also showed a great increase in soil phosphorus content from March 2020 to July 2020 (Figure 7(e)).

These high values of phosphorus concentration for these two treatments can be explained by the addition of phosphoric acid as a fertilizer to FW at the beginning of the vegetative development stage of the olive trees because phosphoric acid is a major element for the plant and promotes its growth (Ferreira et al. 2018).

pH trend has also a relevant role to solubilize P (Wang et al. 2007) and/or developing cluster roots, which provide enhanced zones for P uptake. In olive, studies showing a positive tree response to P fertilizers are practically non-existent (Fernández-Escobar et al. 2017). The absence of response may be due to the very low amount of P removed in harvest (Rodrigues et al. 2012; Fernández-Escobar et al. 2017).

Instead, soil P mobility is very low and most P uptake occurs by root interception (Marschner 1995). Therefore, P fertigation improves P availability and enhances the plants' potential to take up P rapidly when required (Zipori et al. 2015). Thus, the TWW application could have contributed to providing a soluble source of P (Zipori et al. 2020).

Regarding soil potassium behavior, the statistical analysis shows that there is no significant difference between the four treatments (p = 0.151). In general, it can be concluded that there was a decrease followed by a slight increase for all treatments except for treatment FW + IN where the concentration of potassium in the soil continued to decrease (Figure 7(f)).

These results can be explained by the fact that in the vegetative development stage, the olive trees consume potassium in a higher way than the other stages: The growth stage (Petousi et al. 2015). It can also be added that the FW + IN treatment is the best treatment that ensures a good assimilation of potassium.

Soil application of a large quantity of fertilizer containing K is responsible for major yield increases in olive orchards that had previously been seriously K deficient (Hartmann et al. 1986). Thus, TWW contributed to K fertilization.

Zipori et al. (2015) also found an increase in K uptake with increasing irrigation levels. K mobility and availability for plant uptake are higher under fertigation compared to the broadcast application (Neilsen et al. 1999). However, this increased uptake cannot be attributed solely to the increase in soil K availability resulting from higher K translocation from the adsorbing complex into the soil solution, as uptake of K from foliar application also depends on water availability (Restrepo-Diaz et al. 2008). Although K is considered to have an important role in plant water balance and carbohydrate assimilation in olives (Erel et al. 2014). Erel et al. (2014) found no correlation between K levels and drought tolerance, including stomatal control mechanisms. In Figure 7(f) we can observe that the T1 – TWW + SIM + IN treatment provided a better K level in the soil compared to the other three treatments, observing variation in K values in the same range over time.

Supplementary Table 8 shows the amounts of water delivered during the experiment based on the SIM irrigation scheduling (T1 – TWW + SIM + IN) and crop water requirement estimated according to the standard FAO 56 method (T2 – TWW + IN, T3 – FW + IN, and T4 – FW + DRIPPERS), as well as the TWW nutrient composition for each month. The values shown to SIM model technology a yearly water savings of up to 12% compared to the FAO 56 method.

Due to the fertilizing properties of the TWW, important amounts of nutrients were received during the irrigation events in both treatments. However, the decrease in the water supply to T1 – TWW + SIM + IN treatment means a slight increase (+4.5%) in the fertilizer demand.

The reduction in commercial fertilizer consumption represents a benefit for the farmers to be included in the CBA. Considering a cost of 0.27 €/kg for ammonium nitrate, 0.69 €/L for phosphoric acid, 0.92 €/kg for potassium sulfate, 0.10 €/m³ for the TWW, and 0.14 € for the average cost of pumping one cubic meter of freshwater at the same location, TWW and TWW + SIM treatments show important yearly water and fertilizers savings compared to the current scenario FW + DRIPPERS (Supplementary Table 9).

These results are similar to the economic gains from fertilizer savings reported by Choukr-Allah & Hamdy (2005) in the region, which ranged from € 350/ha for maize to more than € 780/ha for durum wheat.

Considering the project cost, water and fertilizer cost saving (Supplementary Table 9) and assuming a 12 and 10% yield increase for TWW + SIM and TWW scenarios, respectively, the results of the CBA (Supplementary Table 10) are favorable as the NPV is positive in both scenarios. Negative BCRs also signal feasibility for the options under evaluation and this can be explained by the fact that while the costs have decreased and added to a negative value benefits have increased and added to a positive value.

The values of −1.53 and −1.47 mean that thanks to the project, the costs would generally decrease (as we have cost savings) and the benefits obtained would be equal to 1.53 for each unit of cost saved. The internal rate of return (IRR) shows a value of 48.50 and 46.3%, respectively, which is clear profitability of the project considering the market interest rate of 5%.

Sensitivity analysis carried out for potential yield variation shows (Supplementary Table 11) that three intervals of yield difference can be distinguished:

• Yield decreases greater than 46%: The NPV is negative indicating that the project is not able to cover the costs and will lose money. The BCR will also continue to increase beyond the value of one. Since the costs are negative, this means that it will be losing more than 1 unit of benefits for 1 unit of costs saved, also pointing out that the project is not feasible and should not be conducted.

• Yield decreases between 23 and 46%: The NPV will continue to increase from zero to €17,133. This means that the project will start making money. The BCR will also decrease from one to zero. Given that the costs are still negative, this means that one unit of costs saved corresponds to a value (between zero and 1) of benefits lost. This project can be considered cost-effective and should be pushed forward.

• At yield increase rates or yield decrease of less than 23%, the NPV is positive and the BCR is lower than −1. This means that the project would allow making benefits and saving costs at the same time. The value is less than one meaning that one unit of costs saved can generate benefits even higher than that.

Morocco, just like the neighboring Mediterranean countries, is facing a number of challenges in water resources management. As the water demand tends to increase and the irregularity of precipitations becomes more acute, alternative water resources have been considered to help mitigate these effects. Wastewater is one of the most considerations even though only 25% of the wastewater production in Morocco was treated and an even smaller portion (11%) was reused (Mansir et al. 2021).

Social acceptance, potential risks to the health (possible contaminations), to soil (damage to the structure and physicochemical properties), technical (i.e. irrigation management), and environmental (e.g. groundwater contamination) issues are among the main limiting factors of TWW reuse in agriculture.

This study carried out on olive trees in Morocco tested two innovative technologies: innovative nozzles (IN) and an irrigation practice (SIM) to tackle some of the most important issues related to TWW. An IN (48 l/h) was used as a strategy to reduce the clogging hazards, which commonly occurred with the commercial drippers and combined with an innovative management tool for the TWW irrigation (SIM model) to assess the effects on the soil and olive trees.

The results showed that after 1 year of irrigation with the nozzle technology, the vegetative growth of the olive trees was similar regardless of the irrigation device used. This was also confirmed by the photosynthesis rate measurements and leaf analysis. However, the IN showed a significant difference in DU compared to the commercial drippers.

The soil, however, reacted differently as its alkalinity stayed more or less the same using TWW but witnessed a decrease for the FW treatments. The soil solution EC was generally higher for TWW compared to FW with an upward trend for both.

The P availability was also affected in this case study as the TWW averaged fewer quantities because of the low phosphorus concentration in the TWW of the region.

As no additional constructions and infrastructure investments were needed for the TWW, it did extremely well in the financial analysis. In fact, these costs could be very important and are crucial for the financial viability of related projects (Galvis et al. 2018). The results show that using TWW for olive irrigation could not only bring very important benefits to the farmer but also save costs.

The use of the SIM for irrigation management resulted in 12% water savings compared to the standard management. A difference this significant can be a crucial factor later in the project when the irrigation requirements become much higher for the olive grove.

To conclude, understanding the impacts of TWW irrigation and having the tools to manage it is a crucial part of adapting to reduce the pressure on freshwater in many regions of the Mediterranean basin and particularly in the Souss-Massa region. As such, a long-term study of the impacts of using this water resource is needed to draw solid conclusions and further help in the decision-making.

The authors acknowledge the funding received from the European Union's Horizon 2020 Research and Innovation program (MADFORWATER). They extend their gratitude to the editor and reviewers for their valuable comments and suggestions, which have greatly improved this manuscript. Special thanks go to the managers and staff of the Ocean Golf Course, as well as all the staff at RAMSA at the L'Mzar treatment plant, for their cooperation and assistance.

This project received funding from the EU Horizon 2020 research and innovation program under grant agreement No. 688320 (MADFORWATER project; www.madforwater.eu).

This research did not involve the use of humans or animals.

All relevant data are included in the paper or its Supplementary Information.

The authors declare there is no conflict.